import tkinter as tk
import time

ui = tk.Tk()
ui.attributes("-fullscreen", 1)

def fresh():
    nowTime = time.strftime("%H:%M:%S", time.localtime())
    TimeLabel["text"] = nowTime
    ui.update()
    ui.after(1000, fresh)

TimeLabel = tk.Label(font=("微软雅黑", 100), bg="light grey")
TimeLabel.place(x=200, y=200, width=1136, height=464)

fresh()

ui.mainloop()